Current demands for high density and performance associated with ultra large-scale integration require sub-micron features, increased transistor and circuit speeds and improved reliability. Such demands require formation of device features with high precision and uniformity, which in turn necessitates careful process monitoring, including frequent and detailed inspections of the devices while they are still in the form of semiconductor wafers.
Conventional in-process monitoring techniques employ a two-phase “inspection and review” procedure. During the first phase the surface of the wafer is inspected at high-speed and relatively low-resolution. The purpose of the first phase is to produce a defect map showing suspected locations on the wafer having a high probability of a defect. During the second phase the suspected locations are more thoroughly analyzed. The two-phases of the detection procedure may be implemented by the same defect detection system, but this is not necessarily so.
The two-phased inspection tool may have a single detector or multiple detectors. A multiple detector two-phase inspection device is described in U.S. Pat. Nos. 5,699,447, 5,982,921 and 6,178,257B1 of Alumot (hereinafter collectively referred to as the Alumot system) whose contents are hereby incorporated herein by reference.
The Alumot system has multiple channels for detecting defects. These multiple channels include a plurality of detectors arranged in a circular array around an objective lens.
During the first phase (also referred to as the inspection phase) each channel (a) obtains an inspected pixel, neighborhood inspected pixels, a reference pixel, and neighborhood reference pixels, (b) determines the type of the inspected pixel and/or determines the reference pixel type, (c) compares the inspected pixel and the reference pixel and a threshold that depends upon the inspected and reference pixel types, (d) and determines the presence of a defect in response to said comparison.
The step of determining the type of a pixel involves a first stage of determining the following parameters: (i) local maximum—whether the pixel is a local maximum (if the pixel is a maximum relative to his neighbors), (ii) intensity—if the pixel is intense (if the intensity of the pixel is above a threshold), (iii) ratio—if the ratio between the intensity of a pixel and the intensity of its neighbors is above a threshold and (iv) gradient—whether the pixel is located in a slope area, as determined by the gradient relative to a threshold. The second stage involves classifying the pixel as one of the following types, in response to the parameters: (I) isolated peak (if the pixel is a local maximum with significant intensity and ratio), (II) multi-peak (if the pixel is not an isolated peak, it has significant intensity and none of its neighbors is an isolated peak), (III) slope—if either one of the pixel's neighbors is an isolated peak or has above-threshold gradient, or (IV) background—if the pixel has intensity and gradient below some thresholds and none of its neighbors is an isolated peak.
The outputs (alarm values) of the multiple channels are provided to a decision table that makes a decision regarding a presence of a defect in response to the alarm values. There are three possible alarm values indicating no alarm, low alarm and high alarm.
The decision table outputs a defect flag if (i) at least one out of the eight alarm values is high alarm, and (ii) at least two alarm values from adjacent channels are either low alarm or high alarm.
In the Alumot invention and in similar inspection systems, defect detection takes place in each channel independently: both the typing process and the die-to-die comparison process are performed separately on each data stream. At most the alarm values of suspected defects from each channel are then further processed to minimize false alarms and increase reliability of defect reporting.
It is known in the art that many defects, such as foreign particles, recesses, scratches and shorts can be characterized by their scattering patterns. A light beam that is directed to a defect may be scattered to at least one direction thus defining a scattering pattern. The scattering pattern is at least partially detected by multiple detectors that are arranged in distinct angular locations around the light beam axis. It is noted that the scattering pattern is responsive to various parameters such as, but not limited to, the shape of the defect, the orientation of the defect and the like.
There is a need to provide an improved system and method for defect detection that makes use of the information collected in the different channels in a more sophisticated manner than independent typing and comparison.
There is a need to provide a system and method for defect detection that utilizes signals from multiple collection channels to compare scattering patterns, as well as multi-channel typing and high-speed defect detection.
There is a need to provide a high reliability ultra fast defect detection system and method. There exists a further need for a method for inspecting a substrate that allows a dynamic definition of pixel types.